Dave,
Sorry mate. I guess I'm this forum's 'backpack representative', however, I would have to do a lot of thinking and typing to answer all your questions.
In a nutshell, use setting two (you should have a flow rate of around two-to-two-and-a-half litres per minute; measure it using a bucket, a watch and measure the amount of water produced with a measuring jug).
First cleans with WFP can be a pain; go overboard.
Yes, it's extremely important to rinse.
Now what you should do is use the SEARCH facility at the top of the page and search for 'backpack', 'first WFP cleans' and 'rinsing'.
Also, do a search for 'WFP letter of explanation'.
Once you've spent a couple of hours of reading, then return and ask any questions you've still got.
I don't mean to sound rude, but we keep on 'reinventing the wheel' here.
Good luck with the backpack.
Regards,
Tosh.